It's unknown when or why the world will end, but many people have predicted the date over the years, only to look a little silly when nothing happens. Here's a few of the most famous dates that the world was/is prophesied to end.
The Y2K bug was predicted to be the end of the world on 1st January, 2000. In short, people believed that computers and machines wouldn't be able to handle the change of date from 1999 to 2000, and would malfunction, causing a breakdown of society and disasters worldwide.
21st December, 2012
The Mayan apocalypse was prophesied to occur on 21st December, 2012, due to being the end date of the 5126 year Mayan calendar. Many believed that the cause of the end of the world would be an asteroid crashing into it. There was even a film made about it called 2012.
2020
Self-proclaimed psychic, Jeane Dixon, has predicted that the world will end in 2020. She also claims that Jesus will return in the following years to defeat Satan. Of course, this should be taken with a pinch of salt, as Jeane also predicted that the world would end in 1962.